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Scientists at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) in California (US) have discovered an inorganic halide perovskite that is ideal for smart solar windows. What is more, the new photovoltaic glass is also thermochromic — it produces electricity when the window is darkened at night or in cool conditions.

“We found a good way to control the phase transition of the inorganic perovskite materials and thin films by thermal heating and moisture treatment and we successfully fabricated robust devices,” reports co-lead author Letian Dou, who is now assistant professor of chemical engineering at Purdue University but had been a postdoctoral fellow in Peidong Yang's group, who conducted the study in Berkeley Lab’s Materials Sciences Division. “We utilized the reversible phase transition in the inorganic halide perovskites to produce a new type of solar cell that has two intrinsic characteristic states: one that is highly transparent — and which can thus function as a window — and the other that strongly absorbs light — and which can thus function as a shade.”

Dou notes that smart windows that can transition from transparent to opaque have been around in one form or another since the 1970s. The devices are made from electrochromic, thermochromic and liquid crystal materials, and their transparency is controlled by simply absorbing or reflecting sunlight, without actually converting the solar energy into electricity. “Although photovoltaic windows that can harvest and exploit incoming solar energy are beginning to appear, most of the semiconductors used in these structures cannot be reversibly switched between a transparent phase and a non-transparent phase without deteriorating their electronic properties,” the researcher emphasizes. “Our thermochromic perovskite photovoltaics combine the advantages of both smart windows and photovoltaic windows.”

The potential of inorganic perovskites

The study’s new findings could have an impact on the way different forms of perovskites are used as light-harvesting materials. “Inorganic perovskites are more stable, so they may replace the current organic-inorganic hybrid perovskites,” Dou says. Eventually, the solar materials breakthrough may herald progress in building-integrated photovoltaics (BIPV) and in the way we power buildings altogether, he agrees: “The smart PV windows may replace the regular glass windows to ‘power up’ and ‘cool down’ buildings on sunny days.”

The surprising robustness of inorganic perovskite

The team did not originally set out to develop a thermochromic solar window. Rather, Dou and his colleagues made the discovery while investigating the phase transition in solar cells based on an inorganic halide perovskite, which they then found to have remarkable phase transition chemistry. “We are surprised that the films are quite robust under repeated cycling,” Dou reveals. “The inorganic perovskite material changes its crystal structure between two distinct phases without introducing more defects and degrading its performance.”

Next steps

Continuing their work, the researchers want to better understand the mechanism behind phase transitions. Furthermore, their goal is to enhance the power output when the thermochromic device is in the high-T phase solar cell mode. Lastly, they want to be able to better control the switching process, including lowering the phase transition temperature.